A typical valve needle for a fluid injection valve comprises a needle shaft and a sealing ball welded to a tip of the needle shaft. Both parts are usually made from stainless steel.
WO 91/14864 A1 discloses a solenoid valve with an armature which acts on a valve closure body by means of a connecting tube. The metallic valve closure body is first provided with a spot weld which is then welded to the metallically softer connecting tube by a second spot weld until the at least partially surrounding weld seam which ensures a safe and reliable connection is formed between the valve closure body (35) and the connecting tube. The welding is conventionally performed by laser welding, where the parts are rotated.
Conventionally, laser welding requires a tooling for maintaining both parts in contact, a rotating motor spindle and a beam laser transporter, which is commonly an optic fiber and focus optical group. To weld the sealing ball to the needle, the parts are pressed, put into a synchronized and coaxial rotation and then the laser welding is performed by a stationary laser. Such an axial-symmetric laser welding can be affected by parts distortion due to thermal stress and/or excessive eccentricity due to the rotation of the parts during welding. To prevent this, document DE 10 2005 000 631 A1 suggests moving the laser beam around the stationary workpiece by using a mirror arrangement. This is, however, a rather complicated setup and does not solve problems related to thermal stress during the welding process.